The incompatibility of urea-formaldehyde (UF), including melamine urea formaldehyde (“MUF”), based binders with cereal straws is reflected in current commercial ventures making panels from these materials. All conventional prior art strawboard plants use methyl diphenyl isocyanate (“MDI”) as the binder in an effort to make particleboard and MDF. While MDI is an excellent binder, which imparts superior properties to panels, MDI has some inherent disadvantages, including its high cost, for non-wood non-structural panels.
One significant disadvantage is the tendency of MDI to adhere to press platens during the pressing process. A variety of release techniques are available to overcome the bonding of MDI to press platens, such as release agents and release papers. However, when compared to UF-based resins, the use of internal and external release agents and release papers is expensive and add to the cost of the end product. Another often overlooked deficiency of MDI, when used in combination with cereal straws, is lack of mat tack which is a critical issue in the preparation of straw based non-structural panels.
Lower binder costs, lower process costs, increased ease of implementation and better mat integrity all provide the incentive to use UF-based binders with straw in panels. The barrier has been the inability to bond UF binders with straw to exceed minimum commercial standards.
There have been various theories proposed on why UF does not bond with straw in composite panels. Incompatibilities between a straw's waxy epicuticular layer and water-based resins, straw silica content and straw chemical reactivity, separately or in unison, are believed to be the reasons why UF based resins cannot be effectively used with straw. However, straw-UF bonding knowledge is somewhat limited according to published reports.
A series of studies conducted at Oregon State University (Groner and Barbour, 1971, 1972, 1973) found that MDI was the most effective binder with straw and that binder effectiveness could be increased by chemically stripping the wax from straw. Such a gain in bondability was at the expense of thickness swell, however. In the establishment of commercial straw panel board plants, subsequent piloting trials have demonstrated in practice that MDI is superior in performance over UF. This further reinforces the idea of UF-straw incompatibility. This idea is so widely held that there are no commercial straw panel ventures that presently use UF resins.
Research conducted at Washington State University indicated that pressure refined straw displayed increased UF resin bondability over hammermilled straw (Sauter, 1995). The results were still well below the commercial standard however. Of note was the change in buffering capacity of straw to something more closely resembling the buffering capacity of wood, when the straw was pressure refined. This decrease in the straw buffering capacity was postulated to be the most likely reason why increased UF bonding was encountered. The mechanism causing this change in buffering capacity was considered to be the development of weak acids in the refining process.
A chemi-thermomechanical treatment was disclosed in WO99/02318 (Nakos) whereby an acid (or alkaline) wash is applied to the straw. It is claimed the wax and silica are removed from the straw by the combined chemical and mechanical action thus facilitating the UF bonding process. Again, the results indicated improvement in bond only but not exceptional bonding performance, nor sufficient bonding performance to meet commercial standards.
In U.S. Pat. No. 5,656,129, a method of producing straw fibre is disclosed which utilizes a steam contacting and steam pressure refining steps. Steam pressures up to 100 psig (6.89 bar) are disclosed while pressures of 40 to 75 psig (2.76 to 5.17 bar) are preferred. In the prior art, pressures above about 6 bar are not taught because of two reasons. First, the darkening of the fibre results in end products which are cosmetically unsuitable. Second, it is known that straw fibre length decreases as steam refining pressure increases, which is not considered desirable.
Therefore there is a need in the art for improved methods of processing cereal straws to form panels using UF and MUF resins, because of the potential advantages of using UF and MUF resins.